1. Field of the Invention
The present invention relates to an In-Plane Switching (IPS) mode liquid crystal display (LCD) device, and more particularly, to an IPS mode LCD device to decrease a common voltage drop and delay.
2. Discussion of the Related Art
A liquid crystal display (LCD) device is a flat display. The LCD device changes optical anisotropy by applying an electric field to a liquid crystal layer. The LCD device has been widely used as it has a number of advantageous characteristics as compared with a conventional display using a cathode ray tube (CRT). These characteristics include low power consumption, thin profile, high resolution, and suitableness for a large size.
The LCD device includes an LCD panel for displaying images, and a driving circuit part for supplying driving signals to the LCD panel. Also, the LCD panel includes first and second substrates bonded to each other at a predetermined interval therebetween, and a liquid crystal layer formed between the first and second substrates by injection of a liquid crystal material.
At this time, the first substrate (thin film transistor array substrate) includes a plurality of gate lines arranged in a first direction at fixed intervals, a plurality of data lines arranged in a second direction perpendicular to the gate lines at fixed intervals, a plurality of pixel electrodes in respective pixel regions defined by the gate lines and the data lines arranged in a matrix-type configuration, and a plurality of thin film transistors (TFTs) being switchable in response to a signal of the gate line for transmission of a signal of the data line to the pixel electrode. Also, the second substrate (color filter array substrate) has a black matrix layer for shielding light from areas excluding the pixel regions, and an R/G/B color filter layer for displaying various colors. A predetermined interval is maintained between the first and second substrates by spacers, and the first and second substrates are bonded to each other by a sealant having an injection inlet. Then, the liquid crystal material is injected between the first and second substrates.
The LCD device has various modes according to the properties of the liquid crystal used and the pattern structure. More specifically, the LCD device is categorized into a Twisted Nematic (TN) mode for controlling liquid crystal director by applying a voltage after arrangement of liquid crystal director twisted at 90°, a multi-domain mode for obtaining a wide viewing angle by dividing one pixel into several domains, an Optically Compensated Birefringence (OCB) mode for compensating a phase change of light according to progressing direction of light by forming a compensation film on an outer surface of a substrate, an In-Plane Switching (IPS) mode for forming an electric field parallel to two substrates by forming two electrodes on any one substrate, and a Vertical Alignment (VA) mode for arranging a longitudinal (major) axis of liquid crystal molecule vertical to a plane of an alignment layer by using negative type liquid crystal and vertical alignment layer.
Among these types of LCD devices, the IPS mode LCD device has a pixel electrode and a common electrode formed on the same substrate. That is, the IPS mode LCD device includes a first substrate having gate and data lines crossing each other to define a pixel region, a switching device formed at a crossing point of the gate and data lines, and the common and pixel electrodes alternately formed to generate an electric field parallel to the two substrates.
Hereinafter, a related art IPS mode LCD device and a method for fabricating the same will be described with reference to the accompanying drawings.
FIG. 1 is a plane view of a unit pixel of an IPS mode LCD device according to the related art. FIG. 2 is a voltage distribution of an IPS mode LCD device along I-I′ of FIG. 1. FIG. 3A and FIG. 3B are plane views of an IPS mode LCD device when a voltage is turned on/off.
That is, as shown in FIG. 1, a thin film transistor array substrate 11 includes a gate line 12, a data line 15, a thin film transistor TFT, a common line 25, a plurality of common electrodes 24, a plurality of pixel electrodes 17, and a capacitor electrode 26. At this time, the gate line 12 is formed along one direction on the thin film transistor array substrate 11, and the data line 15 is formed perpendicular to the gate line 12 to define a pixel region. Also, the thin film transistor TFT is formed at a crossing portion of the gate and data lines 12 and 15. Then, the common line 25 is formed parallel to the gate line 12 within the pixel region. The plurality of common electrodes 24, diverging from the common line 25, are formed parallel to the data line 15 within the pixel region. Also, the plurality of pixel electrodes 17 are connected with a drain electrode of the thin film transistor TFT, and each of the pixel electrodes 17 is provided between the common electrodes 24 in parallel. The capacitor electrode 26 extending from the pixel electrode 17 is formed on the common line 25.
In addition, the thin film transistor TFT is comprised of a gate electrode 12a diverging from the gate line 12, a gate insulating layer (not shown) formed on an entire surface of the thin film transistor array substrate 11 including the gate electrode 12a, a semiconductor layer 14 formed on the gate insulating layer above the gate electrode 12a, and source and drain electrodes 15a and 15b diverging from the data line 15, and formed at both sides of the semiconductor layer 14.
At this time, the common line 25 may be formed as one with the common electrode 24, and the gate line 12 may be formed as one with the gate electrode 12a. Also, the common line 25 and the gate line 12 may be formed of a low-resistance material at the same time.
The pixel electrodes 17 are formed of a transparent conductive metal material having great transmittance, for example, indium-tin-oxide (ITO). Each pixel electrode 17 alternates with the common electrode 24. Also, the pixel electrode 17 is in contact with the drain electrode of the thin film transistor TFT, whereby the pixel electrode 17 receives a voltage.
Also, on the common line 25, the capacitor electrode 26 is formed as one with the pixel electrode 17, thereby forming a storage capacitor.
In the related art IPS mode LCD device, as shown in FIG. 2, if 5V is applied to the common electrode 24, and 0V is applied to the pixel electrode 17, an equipotential surface is formed in parallel to the electrodes at the portions right on the electrodes, and the equipotential surface is formed nearly perpendicular to the electrodes at the portion between the two electrodes. Thus, since an electric field is perpendicular to the equipotential surface, a horizontal electric field is formed between the common electrode 24 and the pixel electrode 17, a vertical electric field is formed on the respective electrodes, and both the horizontal and vertical electric fields are formed in the edge of the electrode.
An alignment of liquid crystal molecules in the related art IPS mode LCD device is controlled with the electric field. For example, as shown in FIG. 3B, if a sufficient voltage is applied to liquid crystal molecules 31 initially aligned at the same direction as a transmission axis of one polarizing sheet, long axes of the liquid crystal molecules 31 are aligned in parallel to the electric field. In case the dielectric anisotropy of the liquid crystal is negative, short axes of the liquid crystal molecules are aligned in parallel to the electric field.
More specifically, first and second polarizing sheets are formed on outer surfaces of the thin film transistor array substrate and the color filter substrate bonded to each other. The transmission axes of the first and second polarizing sheets are perpendicular to each other. Also, an alignment layer formed on the lower substrate is rubbed in parallel to the transmission axis of one polarizing sheet. This sets the display in a normally black mode.
That is, if the voltage is not provided to the LCD panel, as shown in FIG. 3A, the liquid crystal molecules 31 are aligned to display the black state. As shown in FIG. 3B, if the voltage is provided to the LCD panel, the liquid crystal molecules 31 are aligned in parallel to the electric field, thereby displaying the white state.
An operation of the related art IPS mode LCD device will be described as follows.
The general LCD devices inclusive of the IPS mode LCD device have the pixel regions arranged in the matrix-type configuration. In the general LCD devices, the video signal is applied to the corresponding pixel region when a scanning signal is inputted to one gate line.
The liquid crystal material injected between the first and second substrates may deteriorate when a DC voltage is applied for a long time. In order to prevent such a problem, a polarity of the supplied voltage is cyclically changed, which is commonly referred to as a polarity inversion method. The polarity inversion method is classified into a frame inversion method, a line inversion method, a column inversion method, and a dot inversion method.
In the frame inversion method, positive and negative polarities of a data voltage supplied to the liquid crystal material for a common electrode voltage are alternately supplied during each frame. For example, if a positive (+) polarity data voltage is supplied to an even frame, a negative (−) polarity data voltage is supplied to an odd frame. Thus, the same polarity data voltage is supplied according to the even or odd frame, thereby decreasing consumption current during a switching mode. However, the frame inversion method is sensitive to flicker generated according to an asymmetrical transmittance between the positive and negative polarities. In addition, the frame inversion method is susceptible to crosstalk caused by interference between data signals of adjacent pixels.
The line inversion method is commonly used for low-resolution devices (i.e., VGA and SVGA), in which a data voltage is supplied such that a polarity of a data voltage supplied to the liquid crystal material for a common electrode voltage is changed according to a vertical direction. For example, in a first frame, a positive (+) polarity data voltage is supplied to an odd gate line, and a negative (−) polarity data voltage is supplied to an even gate line. Next, in a second frame, the negative (−) polarity data voltage is supplied to the odd gate line, and a positive (+) polarity data voltage is supplied to the even gate line. In the line inversion method, the polarities of the data voltage are oppositely supplied to adjacent lines such that a luminance difference is offset between the lines according to spatial averaging, thereby preventing the flicker during frame inversion. For example, the opposite-polarity data voltages are supplied along a vertical direction, so that a coupling phenomenon of the data signals is offset, thereby decreasing vertical crosstalk during the frame inversion. However, the polarity of the data voltage is the same along a horizontal direction, so that horizontal crosstalk is generated, and consumption current is increased due to an increase of the number of switching operations, as compared with that during the frame inversion.
In the column inversion method, the same polarity of a data voltage supplied to liquid crystal material for a common electrode voltage is supplied in the vertical direction, and positive and negative polarities of the data voltage are alternately supplied along the horizontal direction. Thus, it is possible to minimize flicker by spatial averaging and to minimize horizontal crosstalk. However, the column inversion method requires a high-voltage column drive IC since the opposite-polarity data voltages are supplied to the adjacent lines according to the vertical direction.
The dot inversion method is supplied to high-resolution devices (i.e., XGA, SXGA, and UXGA) for obtaining the greatest quality picture image. In the dot inversion method, a polarity of a data voltage is differently supplied to all-direction adjacent pixels. Accordingly, it is possible to minimize flicker by spatial averaging. However, the dot inversion method is problematic since the dot inversion method has a high consumption current by using a high-voltage source driver.
Hereinafter, a related art IPS mode LCD device of a dot inversion method will be described as follows. FIG. 4 is an equivalent circuit view of an IPS mode LCD device according to the related art. FIG. 5 is a timing view of a pixel voltage in each gate line of FIG. 4.
In a unit pixel of the related art IPS mode LCD device, a plurality of thin film transistors TFT are formed at respective crossing portions of gate lines (G1, G2, G3, . . . ) and data lines (D1, D2, D3, . . . ). Also, each of common lines (Vcom1, Vcom2, Vcom3, . . . ) is formed between each of the gate lines. Then, a storage capacitor Cst and a liquid crystal capacitor CLC are formed in parallel between a pixel electrode (‘17’ of FIG. 1) connected with a drain electrode in each thin film transistor and the common line.
At this time, the polarity of a data voltage applied to each data line is inversely applied to respective pixels by each horizontal period. That is, the data voltage is applied such that positive (+) and negative (−) polarities are inversely applied to the respective pixels by each horizontal period. When a gate pulse is applied to the corresponding gate line so as to drive the pixel, the thin film transistor of the corresponding gate line is turned-on, whereby the data voltage is applied to the pixel electrode. Then, the liquid crystal capacitor CLC and the storage capacitor Cst, connected between the drain electrode of the thin film transistor and the common line (the pixel electrode and common line), are charged during turning-on the thin film transistor. After turning-off the thin film transistor, electric charges are maintained until the thin film transistor is turned-on.
At this time, as shown in FIG. 5, a DC voltage having an intermediate level between the positive (+) and negative (−) polarities of the data voltage is applied without regard to the pixel, the gate line, or the frame.
Accordingly, a pixel voltage is changed by a difference amount ΔVp according to a parasitic capacitor Cgs formed between gate and source electrodes of the thin film transistor along a falling edge of a scanning signal supplied to the gate line. Then, the pixel voltage having a falling value by the difference amount ΔVp is induced to the pixel electrode.
However, when driving the related art IPS mode LCD device, a constant DC value is supplied as a common voltage, and the positive (+) and negative (−) polarity data voltages for the common voltage signal are alternately supplied to the data lines of the respective pixels. Thus, the pixel voltage Vp supplied to the liquid crystal has the polarity dependent on the data voltage, so that it is required to use a source driver having a great output voltage difference to induce a high voltage to the liquid crystal material, thereby increasing the cost.
In the related art IPS mode LCD device, the liquid crystal is driven according to a fringe field formed between the pixel electrode and the common electrode. Accordingly, it is required to form the fringe field having a great value by narrowing an interval between the pixel electrode and the common electrode.
In order to narrow the interval between the pixel electrode and the common electrode, it is necessary to pattern the pixel and common electrodes in a finger type crossed at a predetermined interval when patterning the pixel and common electrodes. However, if the interval between the pixel electrode and the common electrode becomes narrow, an aperture ratio of the pixel becomes low. To improve the aperture ratio, the pixel or common electrode may be formed of a transparent material, such as ITO (Indium-Tin-Oxide). However, patterns of the pixel and common electrodes having various shapes are formed within the pixel region so that it is difficult to uniformly transmit the light therethrough. When widening the interval between the pixel electrode and the common electrode for improving the aperture ratio, the electric field parallel to the substrates decreases between the pixel electrode and the common electrode. Thus, in order to obtain the required luminance, a data voltage of a high output range is required.
Recently, an IPS mode LCD device and a method for driving the same, to obtain the high liquid crystal voltage between the common electrode and the pixel electrode without using the high output source driver, and to improve picture quality with swing of the common voltage, by supplying the data voltage and the common voltage of the opposite polarity to the odd/even numbered common lines, for the increase of electrode interval and the drop of driving voltage, has been proposed.
FIG. 6 is an equivalent circuit view of a related art IPS mode LCD device for the increase of electrode interval, and the drop of driving voltage. FIG. 7 is a timing view of a pixel voltage in each gate line of FIG. 6.
That is, as shown in FIG. 6, a plurality of gate lines (G1, G2, G3, G4, . . . ) are perpendicular to a plurality of data lines (D1, D2, D3, D4, . . . ). Also, each of common lines (Vcom1, Vcom2, Vcom3, . . . ) is formed between the gate lines, and a thin film transistor TFT is formed at a crossing portion of the gate and data lines. In addition, a storage capacitor Cst and a liquid crystal capacitor CLC are formed in parallel, between the common line and a pixel electrode (‘17’ of FIG. 2) connected with a drain electrode of the thin film transistor.
For the increase of the electrode interval and the drop of the driving voltage in the related art IPS mode LCD device, when a first common voltage (or second common voltage) is applied to the odd numbered common line (Vcom1, Vcom3, . . . ), a second common voltage (or first common voltage) is applied to the even numbered common line (Vcom2, Vcom4, . . . ). In this state, the data voltage of the same polarity is applied to the pixels connected with the same common line.
That is, as shown in FIG. 7, if the data voltage of positive (+) polarity is applied to a predetermined pixel, the first common voltage (Vcom(−)) is applied to the corresponding common line. Meanwhile, if the data voltage of negative (−) polarity is applied to a predetermined pixel, the second common voltage (Vcom(+)) is applied to the corresponding common line.
Accordingly, a voltage difference increases between the pixel electrode and the common electrode.
To apply the common voltage together, the odd numbered common lines are connected with one another at one side by a first common voltage supplying line 1, and the even numbered common lines are connected with one another at one side by a second common voltage supplying line 2. Then, the other side in each common line is in the open state.
However, the related art IPS mode LCD device for swing of the common voltage has the following disadvantages.
That is, when separately applying the common voltage to the odd/even numbered common lines, the odd numbered common lines are connected at one side by the first common voltage supplying line, and the even numbered common lines are connected at one side by the second common voltage supplying line. However, the other side in each common line is in the open state, so that the resistance difference of the common lines generates dependent on the position of the common lines. As a result, the common voltage drops due to the resistance difference, and the RC delay increases, thereby deteriorating the picture quality.